Uses third-party call control to set up a call between two far parties,
See constructor for parameters.
Note that Cisco gateways do not appear to support this call flow acting
as UA A. When tested with c3640-is-mz.123-14.T7 and
c3825-advipservicesk9-mz.124-11.T2, the Cisco gateway responds with
(1) INVITE offer1 no media with reponses with SDP answer that contains
media. After Controller sends (3) ACK with no SDP, the gateway sends BYE.
This fragment uses RFC 3725 Third Party Call Control (3pcc)
call establishment flow IV to establish a call between two
parties.

The required SDP manipulations from offer2 to offer2' and answer2' to answer2
are not implemented. In practice, most UAs do not check the origin line
in the SDP.

Error handling described in RFC 3725 sec 5 are not fully implemented.
For example, the re-INVITE glare error condition in Figure 5 is not
handled. In this glare condition, UA A sends re-INVITE after step (5),
this creating a re-INVITE glare with (6).
There is a problem with the recommendation that the
controller sends a 491 response. Proper handling of this case requires
more investigation.